One of the main requirements imposed on biuret-structure polyisocyanates resides in the necessity of having optimum content of isocyanate groups which defines the product viscosity. The higher the content of isocyanate groups in the final product (greater amount of the reaction centres necessary for the formation of a protective coating), the lower the viscosity (which lowers the amount of a solvent in the preparation of paint and varnish compositions) and the higher is the product quality. The maximum theoretically possible content of isocyanate groups in the desired product is 26.36%. Practically it is smaller and varies from 21 to 24% depending on the process of manufacture.
Known in the art is a method for preparing biuret-structure polyisocyanates by way of reacting an aliphatic diisocyanate such as 1,6-hexamethylenediisocyanate with water (cf. FRG Patent No. 1,101,394). The process is conducted in two stages. In the first stage water is gradually charged into the starting diisocyanate and the mixture temperature is maintained at about 100.degree. C. This stage is performed for a period of from 3 to 6 hours. As a result, a substituted urea is obtained. After the admission of the whole amount of water, the reaction mass temperature is elevated to 130.degree.-140.degree. C. or even up to 250.degree. C. and stirring is continued for one additional hour to give a biuret-structure polyisocyanate. Then the resulting mass is filtered-off from the insoluble urea, the amount of which is 1 to 5%. The filtered-off product is subjected to vacuum distillation to remove the excessive 1,6-hexamethylenediisocyanate.
One of the disadvantages inherent in this prior art method resides in the formation of a by-product therein (insoluble urea) in an amount of from 1 to 5%. This has a detrimental effect on the desired product yield. Besides, the duration of the process of preparation of the polyisocyanate is rather long.
More effective is a single-stage method for producing a biuret-structure polyisocyanate (cf. USSR Inventor's Certificate No. 368282). In accordance with this process, gradual admission of water into the starting diisocyanate such as 1,6-hexamethylenediisocyanate, is effected at a temperature within the range of from 130.degree. to 145.degree. C., preferably from 135.degree. to 138.degree. C. In doing so, to minimize the amount of the forming by-product (polyurea in an amount of up to 0.01%), water is charged in the form of an aqueoacetone solution (I:I) at a rate of from 0.5 to 5 kg/hr per one m.sup.3 of the isocyanate volume. The reaction is proceeded to a content of isocyanate groups of from 32.5 to 34.5%. The resulting reaction mass is subjected to filtration on cartridge-type filters in order to remove the formed polyurea from the desired product and further subjected to vacuum distillation to remove the excessive 1,6-hexamethylenediisocyanate. The thus produced polyisocyanate having biuret structure has a molecular mass of from 700 to 850 (by the cryoscopic method) and a content of isocyanate groups is 21-23%.
This method also necessitates filtration to remove polyurea from the product. Therewith, despite the fact that the content of polyurea per se is small (0.01%), losses of isocyanates (1,6-hexamethylenediisocyanate and biuret-structure polyisocyanates) upon filtration are practically as high as 2% as calculated for the intermediate product. This is due to the fact that polyurea is liable to a strong swelling with isocyanates. Replacement of the spent filtering aid is also accompanied by losses of polyisocyanate.
This method also requires a long time for performing same. The synthesis of a biuret-structure polyisocyanate lasts for 4 to 24 hours. The process technology does not make it possible to provide a continuous method. Furthermore in the process use is made of hazardous acetone for dilution of water, through acetone per se is not a reactant in the process.
During the supply of a solution of water in acetone, the latter, while passing into 1,6-hexamethylenediisocyanate and the reaction products thereof heated to the temperature of 135.degree. C., is evaporated and vented to atmosphere thus increasing, by 2 to 2.5%, the total losses of the starting materials.